Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Flyboy on October 28, 2004, 07:16:30 PM
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i have just thought of something (scary isnt it?)
i know AH cockpits are not exactly realistic, but anyway.
why do we have 2 types of compasses in the cockpit? was it like that in RL planes?
allso how does the vertical one (the one that looks like a clock) work?
please correct me if im wrong
the horizontal one is a simple "floating" compass right?
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Don't quote me on this, but I believe one is magnetic and the other is electrically done. You'd need to ask someone else for a definate answer though.
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You're quite right United.
Flyboy, Im not sure there are 'floating' compasses onboard, what if they'd make a loop or something? It might become stuck that way.
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one is gyroscopic IIRC, and one magnetic.
of course magnetic north is not real north, so that's what the 2 are for again if i recall correctly.
i was told once the "tape" or floating compass is the gyro, and the "dial" or clock is the magnetic.
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the gyro is calibrated after the magnetic compass before you take off.
so that means the gyro will also indicate magnetic headings
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so.... Why 2 compasses?
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In case one fails.
You lose your compass then you better know where you're flying like the back of your hand.
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Nowadays modern airlines have 2 too, one electrical, and the other a 'classical' one, in case of an electrical failure
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In real aircraft the Directional Gyro or DG is set to the magnetic compass (magnetic north) before takeoff. The reason to have a DG is that the "whiskey" compass is prone to a number of errors when turning to and from northerly and southerly headings or accelerating or deccelerating on easterly or westerly headings. The DG shows actual heading changes instantaneously.
The only cockpit oddity specific to the game that I can bust HT on is the fact that the whiskey compass works "backwards" in the game. The compass card in real aircraft is more or less stationary while the aircraft rotates around it.
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BEARING-DISTANCE-HEADING INDICATOR The bearing-distance-heading indicator (BDHI) (fig. 5-5) may be used with the various navigational systems, and it provides information according to the mode selected. Some aircraft may have more than one BDHI, with separate select switches for each instrument. The distance counter numerals may be in a vertical row or horizontal, as shown in figure 5-5. Indicator Parts This section will explain the various parts of the BDHI. Refer to figure 5-5 while reading the following text. The lubber index is a fixed reference mark that allows the operator to read the heading from the Figure 5-5.-Bearing-distance-heading indicator. 5-4 (http://www.tpub.com/content/aviation/14030/img/14030_112_1.jpg)
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I really hate the AH compass and I've never seen the like of it on a plane. All modern planes have the rotating angles disc and a stationary pointing needle like posted in the pic above. It's much easier to read asimuth from it. If you spot bandits at 3 oc. you look at the dial at 3 oc to read the direction to them.
I don't recall seeing non magnetic compasses in WWII cockpits.
The trouble with 2 compasses is which one to belive if they do not agree :)
Bozon
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Compasses in AH are useless. I'm not sure why they bother to model two of them in each plane. You couldn't get lost if you tried.
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In AH the whiskey compass does not work correctly in my opion. Turning to the north from a east or west heading the compass should lead the turn. Turn to the south from a east or west heading the compass should lag the turn. Power advance should dip toward you and turn to the north in a west or east heading. Decrease power it should dip away from you. I think..... Im shure about this, its been awhile thinking about it. To get an accurate compass reading you would have to be level and in unexcelerated flight. But you could not just through a whiskey compass into an airplane and go, it had to be calibrated to electrical and radio interfearence, like gear motors, flap motors, radio transmissions. So after calibration you would have a card mounted next to the compass with calibrated headings so if you flew 180 degrees the card would say fly 183 degrees due to the variation.
The name whiskey compass, comes from when the fluid would leak from the compass, they would use if had to, whiskey to refill the card. It was normally filled with mineral spirits.
Normally the the directional gyro in those days was spun by the venturi system, a venturi tube sat out in the airstream, when air went through it created a low pressure or vaccume which turned the gyro, same for the ADI. Also the DG had to be reset every so often, because it would drift or preset from the heading.
Later airplanes turn the gyros by electricity and the DG would be slaved.
That picture looks close to an early stage of HSI.
Straiga
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a dot heading command would be helpful...now that ah is more gamey
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The compass card in AH II works backwards. It turns the wrong way. I don't think it's a big issue, just kind of interesting. If compass errors were modeled in the compass would lag when turning to or from northerly headings by the approximate value of the current latitude. It would lead when turning to or from southerly headings by the same value. The mnemonic used to remember this is "General Lee leads in the south and lags in the north". When on easterly and westerly headings an acceleration causes the compass to indicate a turn to the north and a decelleration causes the compass to indicate a turn to the south. The mnemonic to remember this is "ANDS" for accelerate north, decelerate south.
The picture above is of an instrument that is referred to today as an RMI (Radio Magnetic Indicator). It consists of a gyroscopic or slaved compass card with needles that reflect the bearing to an NDB or VOR.
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Spitfire I'm curious but since when is a DG system calibrated to your magnetic compass?
Considering a DG system is calibrated using a compass rose and is only recalibrated when either the Magnetic Azimuth Detector is removed and replaced, Major structural changes in the area of the MAD or the compensator assembly fails and requires replacement.
You may slave your card to the magnetic compass but you surely aren't calibrating it. That's the whole point of having both of these seperate instruments. It's so you have 2 independent systems working at the same time. Calibration of the whiskey compass and the DG system are two entirely different jobs.
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Casca,
Your right , now that I look at it closer its is an RMI with a DME.
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A DG (Directional Gyro) can be either slaved to a remote device or be what's called a free gyro. The majority of DGs are free gyros. Free gyros are set to the magnetic heading prior to take off and every 15 minutes or so thereafter due to their tendency to precess (drift off heading). If we want to split hairs the compass heading is not necessarily the magnetic heading, but the magnetic heading uncorrected for deviation (local magnetic disturbances in the aircraft). The compass heading is converted to the magnetic heading by referring to the compass correction card (which is required by FARs). The reason to have a DG is to provide a reliable and instantaneous indication of heading change.
To further murk the waters, DGs up until the 60s or so were not normally vertical presentations. They were horizontal (like a big compass card). I had one once in a 1964 Mooney and it was a pain because you had to do all the math in your head.
To get back to the point of the thread, the second vertical instrument in AH II looks like an ADF (Automatic Direction Finder). In an ADF the needle points to an NDB (non directional beacon) or a commercial broadcast station. The needle on an ADF always points to the station so, like the compass in AH II, the indicator in the game works backwards. If you are flying North (which in the game puts the needle on 0 degrees) and make a 90 degree right turn, the indicator should be on 270 degrees (relative bearing).
So in AH II we have two instruments; one that looks like a whiskey compass (or early DG in some planes) but doesn't really work like one and the other that looks like an ADF but doesn't really work like one.
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Originally posted by Cobra412
Considering a DG system is calibrated using a compass rose and is only recalibrated when either the Magnetic Azimuth Detector is removed and replaced, Major structural changes in the area of the MAD or the compensator assembly fails and requires replacement.
Ummm...no. The DG is adjusted before each flight, as part of the preflight checks. They are a gyroscopic instrument, and subject to precession, and errors induced when they wind down from operating speed. The magnetic compass is calibrated (but not adjusted) and a compensating card supplied, showing the number of degrees of error at a given heading. There is no Magnetic Azimuth Detector (whatever that is) in a Directional Gyro--it's a GYROSCOPE.